Silicon Carbide Single Crystal, Silicon Carbide Single Crystal Wafer, and Method of Production of Same

ABSTRACT

The present invention provides a high resistivity, high quality, large size SiC single crystal, SiC single crystal wafer, and method of production of the same, that is, a silicon carbide single crystal containing uncompensated impurities in an atomic number density of 1×10 15 /cm 3  or more and containing vanadium in an amount less than said uncompensated impurity concentration, silicon carbide single crystal wafer obtained by processing and polishing the silicon carbide single crystal and having an electrical resistivity at room temperature of 5×10 3  Ωcm or more, and a method of production of a silicon carbide single crystal.

TECHNICAL FIELD

The present invention relates to a high resistivity silicon carbidesingle crystal, a silicon carbide single crystal wafer, and a method ofproduction of the same, in particular relates to a high crystal qualitysilicon carbide single crystal, silicon carbide single crystal wafer,and a method of production of the same used for a substrate of a highfrequency electronic device.

BACKGROUND ART

Silicon carbide (SiC) is high in heat resistance and mechanical strengthand further exhibits excellent physical and chemical properties such asresistance to radiation and is therefore coming under attention as anenvironment-resistant semiconductor material. Further, in recent years,demand for SiC single crystal as a substrate material for a blue toultraviolet short wavelength optical device, high frequency high voltageresistance electronic device, etc. has been rising. In applications ofSiC single crystals to the semiconductor field, high quality singlecrystal having a large area is sought. In particular, in applicationsfor substrates of high frequency devices etc., in addition to thequality of the crystal, possession of a high electrical resistance issought.

In the past, on the laboratory scale, for example, SiC single crystal ofa size enabling fabrication of a semiconductor chip has been obtained bythe sublimation recrystallization method (Lely method). However, withthis method, the obtained single crystal is small in area and not easyto control in its dimensions and shape and further in its crystalpolytypes or impurity carrier concentration. On the other hand, anotherpractice has been to use the chemical vapor deposition method (CVDmethod) for heteroepitaxial growth on a silicon (Si) or other differenttype of substrate so as to grow a cubic silicon carbide single crystal.With this method, a large area single crystal is obtained, but thelattice mismatch with the Si substrate is as high as about 20% etc., soit is only possible to grow an SiC single crystal including numerousdefects (up to 107 cm⁻²) and it is not easy to obtain a high quality SiCsingle crystal. To solve these problems, the improved Lely method ofusing an SiC single crystal wafer as a seed crystal for subliminationrecrystallization has been proposed (Yu. M. Tairov and V. F. Tsvetkov,J. Crystal Growth, vol. 52 (1981), pp. 146 to 150). If using thisimproved Lely method, it is possible to grow an SiC single crystal whilecontrolling the crystal polytype (6H type, 4H type, 15R type, etc.) andthe shape, carrier type, and concentration of the SiC single crystal. Atthe present time, SiC single crystal wafers of a size of 2 inch (50 mm)to 3 inch (75 mm) are being cut from SiC single crystal prepared by theimproved Lely method and used for fabrication of devices in theelectronics field etc.

On the other hand, in recent years, as a material for high frequencysemiconductor devices, potassium nitride (GaN) having propertiessuperior even to silicon (Si) or gallium arsenic (GaAs) is coming underattention (Rutberg & Co., Gallium Nitride: A Material Opportunity(2001)). In the fabrication of a GaN device, it is necessary to form aGaN single crystal thin film on some sort of single crystal substrate.As one general type of such a substrate, there is a sapphire substrate.Sapphire has the merit of enabling the stable supply of relatively goodquality single crystal, but has a large difference in lattice constantfrom GaN of 13.8%, so easily induces a deterioration of the quality ofthe thin film formed on it. Further, the thermal conductivity is a small0.42 W/cm·K, so there is also a problem in the point of dissipation ofheat at the time of device operation. A GaN high frequency device formedon a sapphire substrate cannot currently be said to fully realize theinherent performance of GaN in quality and operating properties. Asopposed to this, an SiC single crystal has a small difference of latticeconstant with GaN of 3.4%, so a good quality GaN thin film can beformed. The thermal conductivity is also a large 3.3 W/cm·K, so thecooling efficiency is also high. Compared with sapphire and otherconventional substrates, a great improvement in the properties of GaNdevices can be expected. Therefore, in recent years, expectations havebecome very high for SiC single crystal substrates even in this field.

In high frequency device applications of the above-mentioned substrate,in addition to the quality of the crystal, reduction of the parasiticcapacity of the chips fabricated on it and separation of the chipsrequire that the substrate be raised in resistivity (to at least 5×10³Ωcm, preferably at least 1×10⁵ Ωcm).

At the present time, such SiC high resistivity substrates are beingindustrially obtained by forming deep levels in the bandgap of the SiCsingle crystal by some sort of method. For example, it is known thatvanadium forms deep levels in the SiC crystals in either the donor oracceptor state, compensates for a shallow donor or shallow acceptorimpurity, and raises the crystal's resistivity. Specifically, forexample, as shown in S. A. Reshanov et al., Materials Science Forum,vols. 353 to 356 (2001), pp. 53 to 56, in the above-mentionedsublimation recrystallization method, the SiC crystal powder materialhas added to it metal vanadium or a vanadium compound (silicate, oxide,etc.) which is made to sublimate together with the SiC material andthereby obtain a vanadium-doped crystal. However, the thus prepared SiCsingle crystal has a high resistivity, but is poor in crystal quality.Further, the crystal locations having a high resistivity constitute onlyextremely limited parts in the grown crystal.

Further, Japanese National Publication No. 9-500861 discloses the art ofobtaining a higher resistivity vanadium-doped crystal. This artovercompensates for the impurity nitrogen in the SiC by addition of anelement having a trivalent shallow acceptor level and changes theconductivity type from the n type to the p type to set the vanadium orother transition metal at the donor level and thereby obtain a highresistivity. However, even in the art of this publication, the problemof the vanadium concentration becoming uneven cannot be avoided. Evenwith this art, the inherent problems of crystal quality or yield invanadium-doped crystals are not solved. Further, adding the acceptorelement while controlling it to give the optimum concentration in theSiC crystal is difficult technically. Further, the concentration of theimpurity nitrogen mixed into the SiC crystal in the sublimationrecrystallization method generally changes during the growth by severalorders of magnitude, so maintaining the optimum acceptor elementconcentration in the entire region of the SiC single crystal ingot canbe said to be extremely difficult. For this reason, situations easilyarise where a shortage of the acceptor element makes converting thecrystal in conductivity type to the desired p type impossible oralternatively excessive addition of the acceptor element makes thecrystal become an extreme p type and makes compensation by vanadiumdifficult. The art of this publication also does not solve the inherentproblems of crystal quality and yield in vanadium-doped crystals.

The solid solution limit of vanadium to SiC is about 3 to 5×10¹⁷/cm³ orso. If the amount of vanadium exceeds the solid solution limit, asdescribed in M. Bickermann et al., Materials Science Forum, vols. 389 to393 (2002) pp. 139 to 142, there is the problem that a precipitate formsand the crystal quality drops. The amount of addition of vanadium isalso restricted due to this reason, so in the prior art, highresistivity vanadium-doped crystal was difficult to produce.

On the other hand, it is also known that by reducing the concentrationof the carrier impurity of the SiC single crystal down to a certainextremely low level, the crystal becomes high in resistivity. This isbecause the point defects of the deep levels present in the bandgap ofthe SiC crystals, called ID, UD-1, carbon vacancy, etc. trap theconductive electrons or holes (for example, M. E. Zvanut and V. V.Konovalov, Applied Physics Letters, Vol. 80, No. 3, pp. 410 to 412(2002), B. Magnussen et al., Materials Science Forum, vols. 389 to 393(2002) pp. 505 to 508). However, even the quality of the thus obtainedhigh resistivity single crystal does not satisfy the high requirementsof the semiconductor field at the present time.

DISCLOSURE OF THE INVENTION

In the above explained prior art, when adding vanadium so as to try toraise the SiC single crystal in resistivity, the vanadium concentrationin the crystal has to be made higher than the uncompensated impurityconcentration (|n type impurity concentration other than vanadium—p typeimpurity concentration other than vanadium|). That is, to obtain a highresistivity vanadium-doped crystal, the vanadium concentration has to becontrolled so that the uncompensated impurity concentration <vanadiumconcentration <vanadium solid solution limit (3 to 5×10¹⁷/cm³). However,the concentration of uncompensated impurities of an SiC single crystalis often 1×10¹⁷/cm³ or more. The above-mentioned conditions areextremely narrow in allowable range. Further, vanadium has a sublimationor evaporation speed higher than the sublimation speed of an SiCmaterial, so changes in the vanadium concentration during growth causethe grown SiC crystal to have regions where the vanadium concentrationexceeds the solid solution limit and regions where it is below theuncompensated impurity concentration. For this reason, there were theproblems that conventional vanadium-doped crystal was low in crystalquality and the crystal locations having a high resistivity were onlylimited locations of the grown crystal.

On the other hand, lowering the carrier impurity concentration to raisethe SiC single crystal in resistivity requires the crystal to beincreased in purity to a high level. For this reason, in addition to theuse of a special material, high temperature growth and other specialprocesses become necessary. These are disadvantageous cost-wise.Further, compared with the usual single crystal growth methods, controlof the crystal growth is remarkably difficult, so a high quality crystalcannot be obtained. This point has been a problem.

The present invention solves this problem and provides a highresistivity, high quality, large size SiC single crystal and SiC singlecrystal wafer and a method of production of the same. The inventorsengages in various studies and research for solving this problem and asa result discovered that it is possible to make a crystal high inresistivity by an amount of addition of vanadium far smaller than thepast and thereby provide a high quality and high resistivity SiC singlecrystal. The present invention is configured as follows:

(1) A silicon carbide single crystal containing an uncompensatedimpurity in an atomic number density of at least 1×10¹⁵/cm³ andcontaining vanadium in less than a concentration of said uncompensatedimpurity.

(2) A silicon carbide single crystal as set forth in (1), wherein saiduncompensated impurity has a concentration of not more than 1×10¹⁷/cm³.

(3) A silicon carbide single crystal as set forth in (1), wherein saiduncompensated impurity has a concentration of not more than 5×10¹⁶/cm³.

(4) A silicon carbide single crystal as set forth in any one of (1) (to(3), wherein said uncompensated impurity gives a conductivity type of ann type.

(5) A silicon carbide single crystal as set forth in (1), wherein saidvanadium has a concentration of at least 5×10¹⁴/cm³.

(6) A silicon carbide single crystal as set forth in (1), wherein saidvanadium has a concentration of not less than 1×10¹⁵/cm³.

(7) A silicon carbide single crystal as set forth in (1), wherein saidvanadium has a concentration of not less than 1×10¹⁶/cm³.

(8) A silicon carbide single crystal as set forth in any one of (1) to(7), wherein the difference in concentration of said uncompensatedimpurity and said vanadium is not more than 1×10¹⁷/cm³.

(9) A silicon carbide single crystal as set forth in any one of (1) to(7), wherein the difference in concentration of said uncompensatedimpurity and said vanadium is not more than 5×10¹⁶/cm³.

(10) A silicon carbide single crystal as set forth in any one of (1) to(7), wherein the difference in concentration of said uncompensatedimpurity and said vanadium is not more than 1×10¹⁶/cm³.

(11) A silicon carbide single crystal as set forth in any one of (1) to(10), wherein said silicon carbide single crystal has a main polytype of3C, 4H, or 6H.

(12) A silicon carbide single crystal as set forth in any one of (1) to(10), wherein said silicon carbide single crystal has a main polytype of4H.

(13) A silicon carbide single crystal wafer obtained by processing andpolishing a silicon carbide single crystal as set forth in any one of(1) to (12), wherein said wafer has an electrical resistivity at roomtemperature of at least 5×10³ Ωcm.

(14) A silicon carbide single crystal wafer obtained by processing andpolishing a silicon carbide single crystal as set forth in any one of(1) to (12), wherein said wafer has an electrical resistivity at roomtemperature of not less than 1×10⁵ Ωcm.

(15) A silicon carbide single crystal wafer as set forth in (13) or(14), wherein said silicon carbide single crystal wafer at roomtemperature is a single polytype of 3C, 4H, or 6H.

(16) A silicon carbide single crystal wafer as set forth in (13) or(14), wherein said silicon carbide single crystal wafer is comprised ofa single polytype of 4H.

(17) A silicon carbide single crystal as set forth in any one of (13) to(16), wherein said wafer has a size of at least 50 mm.

(18) A silicon carbide single crystal as set forth in any one of (13) to(16), wherein said wafer has a size of at least 100 mm.

(19) An epitaxial wafer comprised of a silicon carbide single crystal asset forth in any one of (13) to (18) on the surface of which a siliconcarbide thin film is grown.

(20) An epitaxial wafer comprised of a silicon carbide single crystal asset forth in any one of (13) to (18) on the surface of which a potassiumnitride, aluminum nitride, or indium nitride thin film or mixed crystalthin film of the same is grown.

(21) A method of production of a silicon carbide single crystal by usinga sublimation recrystallization method using a seed crystal to grown asingle crystal, said method of production of a silicon carbide singlecrystal characterized by using a sublimating material comprised ofsilicon carbide and vanadium or a vanadium compound in a mixture andusing for the crystal growth a graphite crucible having a nitrogenconcentration of not more than 50 ppm as measured by an inert gas fusionthermal conductivity method.

(22) A method of production of a silicon carbide single crystal as setforth in (21), wherein said graphite crucible has a nitrogenconcentration of not more than 20 ppm.

(23) A method of production of a silicon carbide single crystal as setforth in (21), wherein said graphite crucible has a nitrogenconcentration of not more than 10 ppm.

(24) A method of production of a silicon carbide single crystal as setforth in any one of (21) to (23), wherein said graphite crucible is agraphite crucible treated for purification by being held in an inert gasatmosphere of a pressure of not more than 1.3 Pa at a temperature of1400° C. or more for 10 hours to less than 120 hours.

(25) A method of production of a silicon carbide single crystal as setforth in any one of (21) to (23), further comprising charging thegraphite crucible with a material powder mainly comprised of siliconcarbide and, in that state, treating the graphite crucible forpurification by holding it in an inert gas atmosphere at a pressure ofnot more than 1.3 Pa at a temperature of 1400 to 1800° C. for 10 hoursto less than 120 hours, placing said graphite crucible and seed crystalin an inert gas atmosphere adjusted in pressure to 1.3×10² to 1.3×10⁴Pa, and heating to 2000° C. or more, then starting crystal growth.

(26) A method of production of a silicon carbide single crystal as setforth in (24) or (25), wherein said purification treatment is performedat a pressure of 1.3×10⁻¹ Pa or less.

(27) A method of production of a silicon carbide single crystal as setforth in (24) or (25), wherein said purification treatment is performedat a pressure of 6.5×10⁻² Pa or less.

(28) A method of production of a silicon carbide single crystal as setforth in any one of (24) to (27), wherein after said purificationtreatment, said graphite crucible is used for crystal growth withoutbeing exposed to the atmosphere.

The SiC single crystal of the present invention realizes a higherresistivity by a far smaller amount of vanadium than the past, less thanthe concentration of the uncompensated impurities. This is believed tobe caused by the vanadium and deep level defects being simultaneouslypresent in the SiC single crystal. As a result, there are no longerproblems of the past of excessive addition of vanadium causingprecipitates and inducing deterioration of the crystal quality, and ahigh resistivity can be realized. At the same time, no process forsuperhigh purification of the SiC single crystal which was accompaniedby remarkable difficulties industrially—is needed, so an excessiveincrease in manufacturing costs can be avoided.

In this way, the novel point in the present invention is thesimultaneous introduction of vanadium and deep level defects, which wasnever done together before, into the SiC single crystal and thereby theacquisition of special effects far greater than expected from theeffects in the case of introduction of these independently. Note thatthe present invention is not limited by the mechanism of expression ofthe above-mentioned high resistivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of the configuration of an example of a single crystalproduction system used for production of the crystal of the presentinvention.

FIG. 2 is a view of the analysis positions of an SiC single crystalwafer of a size of 100 mm.

BEST MODE FOR WORKING THE INVENTION

In the SiC single crystal of the present invention, the concentration ofthe uncompensated impurities is preferably 1×10¹⁷/cm³ or less, morepreferably 5×10¹⁶/cm³ or less. The lower limit value of the vanadiumconcentration is 5×10¹⁴/cm³ or more, preferably 1×10¹⁵/cm³ or more, morepreferably 1×10¹⁶/cm³ or more, while the upper limit value is theabove-mentioned concentration of uncompensated impurities. By making theuncompensated impurity concentration and vanadium concentration in theSiC single crystal the above-mentioned values, an effective complexeffect of vanadium and deep level defects is obtained and the crystal ismade high in resistivity. If the concentration of uncompensatedimpurities in the crystal exceeds 1×10¹⁷/cm³ or if the concentration ofvanadium does not reach 5×10¹⁴/cm³, the complex effect of the deep leveldefects and vanadium does not become greater enough to compensate forthe impurity and making the SiC single crystal high in resistivitybecomes remarkably difficult.

Further, by making the difference in concentration between theuncompensated impurity concentration and vanadium concentration in theSiC single crystal 1×10¹⁷/cm³ or less, preferably 5×10¹⁶/cm³ or less,more preferably 1×10¹⁶/cm³ or less, the vanadium and deep level defectsbecome superior over the uncompensated impurity, and the SiC singlecrystal can effectively be made high in resistivity. The type of theuncompensated impurity is not particularly alluded to, but a typicalimpurity of SiC is the donor element nitrogen. When producing an SiCsingle crystal by the sublimation recrystallization method, in generalthe grown crystal most often has a conductivity type of the n type.

When trying to convert the conductivity type to the p type, boron oraluminum or another acceptor element has to be added to the SiC singlecrystal, but avoiding a shortage of the amount of addition or excessiveaddition and producing a p type SiC single crystal of the optimumuncompensated impurity concentration is difficult. In the presentinvention, there is no need to convert the conductivity type to the ptype. The art according to the present invention is more effective whenapplied to an n type SiC crystal. Compared with the concentration of theconditions for obtaining a high resistivity by vanadium in the priorart, the SiC single crystal of the present invention is low in vanadiumconcentration, so either no vanadium compound will precipitate in theSiC single crystal ingot as a whole or even if precipitating, will belimited to local regions, so the crystal quality can be kept high.

Simultaneously, the art according to the present invention enables thecrystal to be made high in resistivity by a broader range ofconcentration of vanadium than the prior art, so all wafers obtainedfrom an SiC single crystal ingot or the majority of the wafers can bemade high resistivity wafers of 5×10³ Ωcm or more, preferably 1×10⁵ Ωcmor more. The SiC single crystal of the present invention can be producedby any polytype of 3C, 4H, and 6H for which application for devices isconsidered promising at present, but among these, in particular, the artof the present invention is most effectively applied to the 4H polytypefor which high device properties can be expected but the amount ofunavoidable nitrogen introduced is large. Further, if consideringapplication as a device, the SiC single crystal wafer is preferablycomprised of a single polytype of either 3C, 4H, or 6H.

As the SiC single crystal ingot for fabricating such a wafer, the ingotas a whole does not necessarily have to be a single polytype, but bymaking the main polytype of the ingot one of the above-mentionedpolytypes, it is possible to improve the yield of the single polytypewafer.

The wafer is not particularly limited in size, but the present inventionis particularly effective in large size SiC single crystal where theplanar distribution of the dopant concentration easily becomes large. Alarge effect is obtained when the size of the single crystal becomes 50mm or more, in particular 100 mm or more. The SiC single crystal waferof the present invention has a high resistivity and further has a highcrystal quality, so can be applied to a high operating frequency device.An epitaxial wafer fabricated by forming an SiC single crystal thin filmon the present invention wafer by the CVD method etc. or an epitaxialwafer obtained by epitaxial growth of a thin film of potassium nitride,aluminum nitride, or indium nitride or a thin film of mixed crystals ofthese is good in crystallinity of the SiC wafer forming the substrate,so has superior properties (thin film surface morphology, electricalproperties, etc.)

To produce the SiC single crystal of the present invention, it ispossible to use a known single crystal growth method such as usingvanadium metal or a silicate, carbide, or other vanadium compound as thesource of vanadium, mixing this with the SiC material by a predeterminedcomposition considering the intake efficiency, and producing the crystalby the sublimation recrystallization method. At that time, as the SiCmaterial, a commercially available material may be used.

In the sublimation recrystallization method, to reduce the concentrationof the uncompensated impurities, it is important to reduce the leadingimpurity, that is, nitrogen, of the SiC crystal. The inventors noticedthat the concentration of nitrogen was often locally high near the seedcrystal of an SiC single crystal grown by the sublimationrecrystallization method and investigated the cause. As a result, theypinpointed the fact that the intermixture into the SiC signal crystal ofthe nitrogen given off from the mainly graphite crucible is one of themain factors behind the reduction of the purity. When using thesublimation recrystallization method using a seed crystal to produce theSiC single crystal of the present invention, by using a graphitecrucible having a nitrogen concentration, as measured by the inert gasfusion thermal conductivity method, of 50 ppm or less, preferably 20 ppmor less, more preferably 10 ppm or less, it is possible to reduce thenitrogen introduced from the crucible into the SiC single crystal andreduce the uncompensated impurity concentration of the SiC singlecrystal.

Further, to obtain such a graphite crucible, it is possible to treat agraphite crucible for purification by holding it in an inert gasatmosphere at a temperature of 1400° C. or more for 10 hours to lessthan 120 hours. At that time, the inert gas atmosphere has a pressure of1.3 Pa or less, preferably 1.3×10⁻¹ Pa or less, more preferably 6.5×10⁻²Pa or less. There is no upper limit on the treatment time, but if thetreatment pressure exceeds 1.3 Pa or the treatment time is less than1400° C., the nitrogen removal efficiency remarkably falls and theheating holding time becomes long, so demerits arise in terms of themanufacturing costs. The treatment temperature is also not particularlylimited, but if the treatment temperature becomes 3000° C. or more,there would be problems in terms of the durability of the treatmentsystem, so this is not preferable.

Further, the above treatment may be performed incorporated in the SiCcrystal growth process. That is, in the sublimation recrystallizationmethod using a seed crystal, the graphite crucible may be filled with asublimating material mainly comprised of SiC and, in that state, may beheld in an inert gas atmosphere at a pressure of 1.3 Pa or less,preferably 1.3×10⁻¹ Pa or less, more preferably 6.5×10⁻² Pa or less, at1400° C. to 1800° C. in temperature for 10 hours to less than 120 hoursso as to treat the graphite crucible for purification. At this time, ifthe pressure exceeds 1.3 Pa, the purification treatment cannot beperformed efficiently.

If the treatment temperature is under 1400° C. also, the purificationtreatment cannot be performed efficiently, while if over 1800° C.,crystal growth ends up starting during the purification treatmentresulting in the formation of polycrystals and making it impossible torealize normal signal crystal growth in the subsequent SiC singlecrystal growth process.

After this purification treatment, without opening the growth furnace tothe atmosphere, the inert gas pressure is adjusted to 1.3×10² to 1.3×10⁴Pa and the crucible is heated to 2000° C. or more for causing SiCcrystal growth. The crystal growth temperature is not particularlydefined in upper limit, but if 3000° C. or more, there are problems inthe durability of the growth system, so this is not preferable. Notethat when not incorporating the graphite crucible purification treatmentin the SiC crystal growth process, if exposing the graphite crucibletreated for purification to the atmosphere, the graphite surfacereabsorbs nitrogen from the atmosphere and the effect of thepurification treatment ends up falling. To prevent the treated cruciblefrom being exposed to the atmosphere, it is possible and more effectiveto make preparations such as filling the sublimating material inside avacuum glove box filled with an inert gas so as to avoid the effects ofreabsorption of nitrogen.

EXAMPLES

Below, examples will be used to explain the present invention morespecifically.

Example 1 and Comparative Examples 1 to 3

Example 1 and Comparative Examples 1 to 3 were produced using thecrystal growth system of FIG. 1. As the seed crystal, an SiC singlecrystal wafer having a size of 50 mm, having a (0001) face, andcomprised of a single 4H polytype was prepared. The seed crystal 1 wasattached to the inside of the graphite lid 4. As the sublimatingmaterial 2, for Example 1 and Comparative Example 1, a commerciallyavailable SiC crystal powder and vanadium compound in mixture werefilled. The vanadium compound was mixed in by an amount giving, byvanadium atom conversion, a mass concentration in the sublimatingmaterial of 0.042%. For Comparative Examples 2 and 3, as sublimatingmaterials, just commercially available SiC crystal powder was filled.Next, the crucible 3 filled with the material was closed by a graphitelid 4 equipped with the seed crystal, covered by graphite felt 7, thenplaced on a graphite support rod 6 and set inside a double wall quartzpipe 5 for crystal growth by the process shown below.

For Example 1 and Comparative Example 3, a crystal growth processincluding treatment of the graphite crucible for purification was used.The details were as follows: First, the inside of the quartz pipe 5 wasevacuated to less than 1.0×10⁻⁴ Pa, then, while continuing to evacuateit, current was run through the work coil 8 to raise the temperature ofthe graphite crucible to the purification treatment temperature of 1600°C. At this time, the internal pressure of the quartz pipe temporarilyrose to 1.3 Pa or more, but while holding the temperature as was, thequartz pipe was evacuated to an internal pressure of the purificationtreatment pressure of 1.0×10⁻¹ Pa or less whereupon treatment was begunto remove the impurity nitrogen in the crucible. The treatment time was48 hours. During that interval, the vacuum evacuation system 11 wasoperated to maintain the internal pressure of the quartz pipe at a valuelower than the above-mentioned value.

After the end of the crucible purification treatment, as the atmosphericgas, high purity Ar gas of a purity of 99.9999% or more was run throughthe pipe 9 under the control of an Ar gas mass flow controller 10 and,while maintaining the pressure in the quartz pipe at the growth pressureof 1.3×10³ Pa, the graphite crucible temperature was raised to thetarget temperature of 2400° C. Thereafter, growth was continued forabout 20 hours. The temperature gradient in the crucible at this timewas 14.5 to 15.5° C./cm, and the growth rate was about 0.8 to 0.9mm/hour. The obtained crystal had a size of about 52 mm and a height of16 mm in the case of the crystal of Example 1 and about 17 mm in thecase of the crystal of Comparative Example 3.

For Comparative Example 1 and Comparative Example 2, the ordinarycrystal growth process not including graphite crucible purificationtreatment was used for the crystal growth. The details were as follows.The inside of the quartz pipe was evacuated to 1.0×10⁻⁴ Pa, then highpurity Ar gas of a purity of 99.9999% or more was run through it. Whileholding the pressure in the quartz pipe at the growth pressure of1.3×10³ Pa, current was run through the work coil 8 to raise thetemperature of the graphite crucible to 2400° C. After this, growth wascontinued for about 20 hours. At that time, the temperature gradient inthe crucible was 14.5 to 15.5° C./cm and the growth rate was about 0.8to 0.9 mm/hour. The obtained crystal had a size of about 52 mm and aheight of 16 mm in the case of the crystal of Comparative Example 1 andabout 18 mm in the case of the crystal of Comparative Example 2.

Before analyzing the obtained SiC single crystal, the graphite crucibletreated for purification treatment was measured for its nitrogenconcentration. First, a graphite crucible of the same material and sameshape as that used for the crystal growth was used for treatment forpurification in the same way as in Example 1 and Comparative Example 3by holding it at a temperature of 1600° C. and a pressure of 1.0×10⁻¹ Paor less for 48 hours. At this time, the graphite crucible was filledwith a sublimating material. After the purification treatment, thegraphite crucible was cooled. The crucible treated for purification inthe vacuum glove box filled with an inert gas was cut to obtain a testpiece of a diameter of about 5 mm and a length of 10 mm as a measurementtest piece. The nitrogen concentration was then measured by the inertgas fusion thermal conductivity method. The measurement was conducted asfollows.

First, the heating crucible in the analysis apparatus was run through bya large current in the state not filled with any sample. The inside ofthe heating crucible was held in the high temperature state of 2500° C.or more for 30 seconds to remove impurity nitrogen from the heatingcrucible surface and vicinity of the surface and make the measurementspace sufficiently high in purity. The heating crucible was cooled, thena test piece kept standing outside the heating zone of the measurementsystem was moved into the heating crucible in a state not exposed to theatmosphere. The inside of the crucible was then held at a hightemperature state of 2500° C. or more for 30 seconds. In this state, thenitrogen produced from the test piece was transported by helium carriergas. The thermal conductivity of this mixed gas was measured todetermine the nitrogen concentration. As a result of the measurement,the crucible treated for purification had a nitrogen concentration ofabout 9 ppm. Further, a graphite crucible not treated for purificationwas measured by a similar method. As a result, the nitrogenconcentration was about 59 ppm.

The thus obtained SiC single crystals of Example 1 and ComparativeExamples 1 to 3 were analyzed by X-ray diffraction and Raman scattering,whereby it could be confirmed that in all ingots, SiC single crystals ofa mainly 4H polytype were grown. To measure the crystals for impurityconcentration and resistivity, three wafers of thicknesses of 1 mm andsizes of 51 were fabricated from each grown single crystal ingot. Theorientations of the wafers were exactly the (0001) faces. The vanadiumconcentration and impurity concentration in the wafer corresponding tothe top, middle, and bottom (vicinity of seed crystal) of each growncrystal (distances from the surface of start of growth of the seedcrystal to the wafer bottom surface of 12 mm, 8 mm, and 4 mm) wereexamined by secondary ion mass spectrometry (SIMS).

According to R. G. Wilson et al., Secondary Ion Mass Spectrometry: APractical Handbook For Depth Profiling And Bulk Impurity Analysis(1989), for analysis of vanadium, a lower limit of measurement of1.5×10¹⁴/cm³ is obtained. In the present invention as well, analysis wasperformed by a method based on this. The lower limit of measurement ofvanadium was less than 5×10¹⁴/cm³. The room temperature resistivities ofthe wafers were examined by the Van der Pauw method. From the aboveanalyses, the results shown in Tables 1 to 4 were obtained.

The uncompensated impurity concentration of the crystal of Example 1(Table 1) was 1.65 to 2.24×10¹⁶/cm³. The main ingredient of theimpurities was the donor element nitrogen. The conductivity due to theimpurity was the n type. The nitrogen concentration after subtractingthe compensation by the impurity acceptor element was the uncompensatedimpurity concentration. Due to the purification treatment of thegraphite crucible, compared with Comparative Example 1 and ComparativeExample 3, the drop in nitrogen concentration, in particular at thebottom of the ingot, was remarkable, so the uncompensated impurityconcentration also fell. The vanadium concentration was lower than theabove-mentioned uncompensated impurity concentration, that is, 3.99×10¹⁵to 3.87×10¹⁶/cm³. The concentration of vanadium in the ingot as a wholedid not exceed the solid solution limit, so no precipitate was formedand the crystallinity was excellent. On the other hand, in all of thewafers at the top and bottom, the resistivity was a high one of 10¹⁰ Ωcmor more.

Comparative Example 1 (Table 2) is a crystal having a vanadiumconcentration substantially the same as the invention example, but inparticular does not utilize the technique of removal of impurities, sothe uncompensated impurity concentration is high. The main ingredient ofthe impurities is nitrogen, and the conductivity due to theuncompensated impurity is the n type. In particular in the region of thestart of growth at the bottom of the ingot, the effect of the nitrogenproduced from the graphite crucible is great. The uncompensated impurityconcentration becomes 1×10¹⁸/cm³ or more. The resistivity is a low oneof less than 1×10⁰ Ωcm throughout the ingot.

Comparative Example 2 (Table 3) is a crystal in which no vanadium wasmixed with the sublimating material. The main impurity is nitrogen, andthe conductivity due to the uncompensated impurity is the n type. Likein Example 1, the technique of treatment of the graphite crucible forpurification is introduced, so the amount of nitrogen mixed in isreduced and the uncompensated impurity concentration is within the rangeof the present invention, but vanadium is not added, so the resistivityis less than 1×10³ Ωcm. Compared with Comparative Example 1, theresistivity is high, but does not reach the required high level.

Comparative Example 3 (Table 4) is a crystal not having vanadium mixedinto the sublimating material and further not using art for reducing theimpurity concentration. The concentration of the main impurity nitrogenis high in the ingot as a whole, in particular the bottom, theconductivity due to the uncompensated impurity is the n type, and theresistivity is a low one of less than 1×10⁰ Ωcm through the ingot.

TABLE 1 Example 1 Room Uncompensated impurity Vanadium temperatureconcentration concentration resistivity Top 1.65 × 10¹⁶ atom/cm³ 3.99 ×10¹⁵/cm³ 3.21 × 10¹¹ Ωcm Middle 2.58 × 10¹⁶ atom/cm³ 8.07 × 10¹⁵/cm³3.59 × 10¹¹ Ωcm Bottom 3.24 × 10¹⁶ atom/cm³ 1.87 × 10¹⁶/cm³ 1.19 × 10¹⁰Ωcm

TABLE 2 Comparative Example 1 Room Uncompensated impurity Vanadiumtemperature concentration concentration resistivity Top 5.95 × 10¹⁷atom/cm³ 3.80 × 10¹⁵/cm³ 9.54 × 10⁻¹ Ωcm Middle 7.22 × 10¹⁷ atom/cm³8.06 × 10¹⁵/cm³ 1.10 × 10⁻¹ Ωcm Bottom 2.40 × 10¹⁸ atom/cm³ 9.97 ×10¹⁵/cm³ 4.36 × 10⁻² Ωcm

TABLE 3 Comparative Example 2 Uncompensated impurity Vanadiumconcentration concentration Resistivity Top 1.24 × 10¹⁶ Less than 5.0 ×10¹⁴/cm³ 5.72 × 10² Ωcm atom/cm³ Middle 2.19 × 10¹⁶ Less than 5.0 ×10¹⁴/cm³ 3.58 × 10² Ωcm atom/cm³ Middle 2.73 × 10¹⁶ Less than 5.0 ×10¹⁴/cm³ 2.17 × 10² Ωcm atom/cm³

TABLE 4 Comparative Example 3 Uncompensated Room impurity Vanadiumtemperature concentration concentration resistivity Top 5.75 × 10¹⁷ Lessthan 5.0 × 10¹⁴/cm³ 1.02 × 10⁻¹ Ωcm atom/cm³ Middle 6.83 × 10¹⁷ Lessthan 5.0 × 10¹⁴/cm³ 5.78 × 10⁻¹ Ωcm atom/cm³ Bottom 2.46 × 10¹⁸ Lessthan 5.0 × 10¹⁴/cm³ 4.24 × 10⁻² Ωcm atom/cm³

Example 2

Next, an example of the process of purification treatment of thegraphite crucible in advance, then crystal growth will be explained.First, a system without the seed crystal and sublimating material ofFIG. 1 was used for purification treatment of the graphite crucible. Thegraphite crucible 3 and lid 4 were covered by the felt 7 and placedinside a double wall quartz pipe 5, the inside of the quartz pipe wasevacuated to less than 1.0×10⁻⁴ Pa, then current was run through thework coil 8 while continuing evacuation so as to raise the temperatureof the graphite crucible and lid to 2500° C. The purification treatmenttime was 20 hours. During that time, the vacuum evacuation system 11 wascontinuously operated to maintain the internal pressure of the quartzpipe at a value lower than the purification treatment pressure of1.3×10⁻² Pa. After the end of the purification treatment, the graphitecrucible 3 and lid 4 were cooled and taken out from the double wallquartz pipe 5 inside a vacuum glove box filled with an inert gas so asto prepare for crystal growth without exposing the crucible to theatmosphere.

As the seed crystal 1, an SiC single crystal wafer having a size of 50mm, having an (0001) face, and made of a single 6H polytype was attachedto the inside surface of the lid 4. A commercially available SiC crystalpowder and vanadium compound in mixture were filled into the graphitecrucible 3 as a sublimating material 2. The vanadium compound was mixedin an amount giving, by vanadium atom conversion, a mass concentrationin the sublimating material of 0.032%. The crucible filled with thematerial was closed by the lid 4 and again placed inside the double wallquartz pipe 5 where the crystal was grown by the next process. Theinside of the quartz pipe was evacuated to 1.0×10⁻⁴ Pa, then high purityAr gas of a purity of 99.9999% or more was run through it. Whilemaintaining the pressure inside the quartz pipe at the growth pressureof 1.3×10³ Pa, current was run through the work coil 8 to raise thetemperature of the graphite crucible to 2400° C. After this, growth wascontinued for about 20 hours. At this time, the temperature gradientinside the crucible was 14.5 to 15.5° C./cm and the growth rate wasabout 0.8 mm/hour. The obtained crystal had a size of about 52 mm and aheight of about 16 mm.

Before evaluating the crystal, for the purpose of confirming the effectof the graphite crucible purification treatment, the nitrogenconcentration of a graphite crucible treated under the same conditionsas the purification treatment of Example 1 was measured by the inert gasfusion thermal conductivity method. The measurement was performed by asimilar method as in the above-mentioned Example 1. The nitrogenconcentration was about 7 ppm.

The thus obtained silicon carbide single crystal was analyzed by X-raydiffraction and Raman scattering, whereby it was confirmed than an SiCsingle crystal having a main polytype of 6H was grown. Three wafershaving orientations of exactly the (0001) face corresponding to the top,middle, and bottom of the grown crystal (distances from the surface ofthe start of growth of the seed crystal of 12 mm, 8 mm, and 4 mm) wereprepared and analyzed in the same way as in Example 1. The results areshown in Table 5.

The main impurity of the crystal of Example 2 (Table 5) was nitrogen,and the impurity conductivity was the n type. The crucible purificationtreatment caused the nitrogen concentration to be greatly reduced. As aresult, the uncompensated impurity concentration fell to 9.76×10¹⁵ to3.01×10¹⁶/cm³. On the other hand, the room temperature resistivitybecame a high value of 1×10¹⁰ Ωcm or more.

TABLE 5 Example 2 Room Uncompensated impurity Vanadium temperatureconcentration concentration resistivity Top 9.76 × 10¹⁵ atom/cm³ 4.75 ×10¹⁵/cm³ 4.12 × 10¹² Ωcm Middle 1.26 × 10¹⁶ atom/cm³ 8.92 × 10¹⁵/cm³6.60 × 10¹¹ Ωcm Bottom 3.01 × 10¹⁶ atom/cm³ 1.97 × 10¹⁶/cm³ 4.74 × 10¹⁰Ωcm

Example 3

Next, an example of production of an SiC signal crystal of the presentinvention having a size of 100 mm or more will be explained. In Example3 as well, the crystal growth system of FIG. 1 was used for theproduction. As the seed crystal 1, an SiC single crystal wafer having asize of 100 mm and (0001) face and made of a single 4H polytype wasattached to the inside surface of the graphite lid 4. The sublimatingmaterial 2, like in Example 1, was comprised of commercially availableSiC crystal powder and a vanadium compound in mixture. The vanadiumcompound was mixed in by an amount giving, by vanadium atom conversion,a mass concentration in the sublimating material of 0.042%. Next, thecrucible 3 filled with the material was closed by a graphite lid 4equipped with the seed crystal, covered by graphite felt 7, then placedon a graphite support rod 6 and set inside a double wall quartz pipe 5for crystal growth by a crystal growth process including graphitecrucible purification treatment. The details were as follows:

First, the inside of the quartz pipe 5 was evacuated to less than1.0×10⁻⁴ Pa, then, while continuing to evacuate it, current was runthrough the work coil 8 to raise the temperature of the graphitecrucible to the purification treatment temperature of 1600° C. At thistime, the internal pressure of the quartz pipe temporarily rose to 1.3Pa or more, but while holding the temperature as is, the quartz pipe wasevacuated to an internal pressure of the purification treatment pressureof 1.0×10⁻¹ Pa or less whereupon treatment was begun to remove theimpurity nitrogen in the crucible. The treatment time was 72 hours.During that interval, the vacuum evacuation system 11 was operated tomaintain the internal pressure of the quartz pipe at a value lower thanthe above-mentioned value. After the end of the crucible purificationtreatment, as the atmospheric gas, high purity Ar gas of a purity of99.9999% or more was run through the pipe 9 under the control of an Argas mass flow controller 10 and, while maintaining the pressure in thequartz pipe at the growth pressure of 1.3×10³ Pa, the graphite crucibletemperature was raised to the target temperature of 2400° C. Thereafter,growth was continued for about 20 hours. The temperature gradient in thecrucible at this time was 14.5 to 15.5° C./cm, and the growth rate wasabout 0.8 to 0.9 mm/hour. The obtained crystal had a size of about 100mm and a height of about 15 mm.

Before evaluating the crystal, the nitrogen concentration of thegraphite crucible treated under the same conditions as the purificationtreatment of Example 3 was measured by the inert gas fusion thermalconductivity method. The measurement was conducted by a method similarto the above-mentioned Example 1, whereby the nitrogen concentration wasfound to be about 8 ppm.

The thus obtained SiC single crystal was analyzed by X-ray diffractionand Raman scattering, whereby it could be confirmed that an SiC singlecrystal of a mainly 4H polytype was grown. Three wafers havingorientations of exactly the (0001) face corresponding to the top,middle, and bottom of the grown crystal (distances from the surface ofthe start of growth of the seed crystal of 12 mm, 8 mm, and 4 mm) wereprepared and analyzed in the same way as in Example 1. For Example 3,for the purpose of examining the planar distribution of the properties,as shown in FIG. 2, the analysis was conducted for a total of fivepoints including the wafer center and four points around it. The resultsof analysis of the center and, as the maximum value of the fluctuation,the maximum value of the analysis value of the center—analysis value ofthe surroundings, are shown in Table 6.

The main impurity of the crystal of Example 3 (Table 6) was nitrogen,and the impurity conductivity was the n type. Along with the larger sizeof the ingot, the volume of the crucible also became larger, so in acrucible with no purification treatment, the amount of nitrogen producedwould increase, but in Example 3, the purification treatment enables thenitrogen of the crucible to be greatly reduced. As a result, the grownSiC single crystal had an uncompensated impurity concentration at thewafer center of a low value of 8.97×10¹⁵ to 3.54×10¹⁶/cm³. While boththe uncompensated impurities and vanadium exhibited some differences ofconcentration in the planes, high room temperature resistivity of 1×10¹⁰Ωcm or more was obtained at the entire surface of the wafer.

TABLE 6 Example 3 Uncompensated Room impurity Vanadium temperatureconcentration concentration resistivity Top Middle 1.76 × 10¹⁶ atom/cm³4.02 × 10¹⁵/cm³ 3.11 × 10¹¹ Ωcm Maximum value of 8.63 × 10¹⁵ atom/cm³3.43 × 10¹⁴/cm³  7.00 × 10⁹ Ωcm fluctuation Middle Middle 2.52 × 10¹⁶atom/cm³ 8.22 × 10¹⁵/cm³ 4.21 × 10¹¹ Ωcm Maximum value of 4.14 × 10¹⁵atom/cm³ 2.80 × 10¹⁴/cm³ 8.50 × 10¹⁰ Ωcm fluctuation Bottom Middle 3.54× 10¹⁶ atom/cm³ 1.94 × 10¹⁶/cm³ 1.24 × 10¹⁰ Ωcm Maximum value of 3.33 ×10¹⁵ atom/cm³ 3.60 × 10¹⁵/cm³  1.10 × 10⁹ Ωcm fluctuation

Example 4

Next, an SiC single crystal ingot produced by the same process as inExample 3 was used to fabricate a mirror surface wafer having anorientation 4 degrees off from the (0001) face in the <11-20> direction,a size of 100 mm, and a thickness of 360 pin. This mirror surface waferwas used as a substrate for SiC epitaxial growth. The growth conditionsof the SiC epitaxial thin film were a growth temperature of 1500° C. andflow rates of silane (SiH₄), propane (C₃H₈), and hydrogen (H₂) of5.0×10⁻⁹ m³/sec, 3.3×10⁻⁹ m³/sec, and 5.0×10⁻⁵ m³/sec. The growthpressure was made the atmospheric pressure. The growth time was 2 hours,whereupon the film grew to a thickness of about 5 μm. The thus obtainedepitaxial thin film was examined under a Normalsky optical microscope,whereupon it could be confirmed that a high quality SiC epitaxial thinfilm having very few pits and other surface defects over the entirewafer surface and having a good surface morphology was formed.

Example 5

Further, another SiC single crystal ingot produced by a process similarto Example 3 was used to fabricate a mirror surface wafer having anorientation of exactly the (0001) face, a size of 100 mm, and athickness of 360 μm. This mirror surface wafer was used as a substratefor epitaxial growth of a potassium nitride thin film by themetal-organic chemical vapor deposition method (MOCVD method). Thegrowth conditions of a gallium nitrogen thin film were a growthtemperature of 1050° C. and flow rates of trimethyl gallium (TMG),ammonia (NH₃), and silane (SiH₄) of 54×10⁻⁶ mol/min, 4 L/min, and22×10⁻¹¹ mol/min. The growth pressure was set to atmospheric pressure.By 60 minutes growth, an n type potassium nitride was grown to athickness of about 3 μm. The thus obtained epitaxial thin film wasobserved under a Normalsky optical microscope, whereupon it could beconfirmed that a high quality potassium nitride epitaxial thin filmhaving an extremely smooth morphology was formed over the entire surfaceof the wafer.

Finally, Table 7 will be used to explain the effects of the presentinvention. Table 7 arranges the above-mentioned Example 1 andComparative Examples 1 to 3 by vanadium concentration and impurityconcentration in the crystal. As shown in Table 7, only InventionExample 1 exhibits the targeted high resistivity. The present invention,which aims at the complex effect of vanadium and deep level defects,gives special effects far greater than anticipated from the effects whenusing these alone and the differences between Comparative Examples 1 and3 or Comparative Examples 2 and 3.

TABLE 7 Comparison of Effects Vanadium concentration 1.0 × 10¹⁵atoms/cm³ or more, less than Uncompensated uncompensated impurityimpurity Less than concentration concentration 5.0 × 10¹⁴ atoms/cm³ 1.0× 10¹⁷ Inventive Example 1 Comparative Example 2 atoms/cm³ bulkresistivity > bulk resistivity < or less 1 × 10¹⁰ Ωcm 1 × 10³ Ωcm 1.0 ×10¹⁷ Comparative Example 1 Comparative Example 3 atoms/cm³ bulkresistivity < bulk resistivity < or more 1 × 10⁰ Ωcm 1 × 10⁰ Ωcm

INDUSTRIAL APPLICABILITY

As explained above, according to the present invention, a highresistivity, high crystal quality SiC single crystal and SiC singlecrystal wafer can be provided. Further, according to the method ofproduction of the present invention, it is possible to keep down therise in manufacturing costs and produce the above SiC single crystal ata high yield.

1. A silicon carbide single crystal containing an uncompensated impurityin an atomic number density of at least 1×10¹⁵/cm³ and containingvanadium in less than a concentration of said uncompensated impurity. 2.A silicon carbide single crystal as set forth in claim 1, wherein saiduncompensated impurity has a concentration of not more than 1×10¹⁷/cm³.3. A silicon carbide single crystal as set forth in claim 1, whereinsaid uncompensated impurity has a concentration of not more than5×10¹⁶/cm³.
 4. A silicon carbide single crystal as set forth in claim 1,wherein said uncompensated impurity gives a conductivity type of an ntype.
 5. A silicon carbide single crystal as set forth in claim 1,wherein said vanadium has a concentration of at least 5×10¹⁴/cm³.
 6. Asilicon carbide single crystal as set forth in claim 1, wherein saidvanadium has a concentration of not less than 1×10¹⁵/cm³.
 7. A siliconcarbide single crystal as set forth in claim 1, wherein said vanadiumhas a concentration of not less than 1×10¹⁶/cm³.
 8. A silicon carbidesingle crystal as set forth in claim 1, wherein the difference inconcentration of said uncompensated impurity and said vanadium is notmore than 1×10¹⁷/cm³.
 9. A silicon carbide single crystal as set forthin claim 1, wherein the difference in concentration of saiduncompensated impurity and said vanadium is not more than 5×10¹⁶/cm³.10. A silicon carbide single crystal as set forth in claim 1, whereinthe difference in concentration of said uncompensated impurity and saidvanadium is not more than 1×10¹⁶/cm³.
 11. A silicon carbide singlecrystal as set forth in claim 1, wherein said silicon carbide singlecrystal has a main polytype of 3C, 4H, or 6H.
 12. A silicon carbidesingle crystal as set forth in claim 1, wherein said silicon carbidesingle crystal has a main polytype of 4H.
 13. A silicon carbide singlecrystal wafer obtained by processing and polishing a silicon carbidesingle crystal as set forth in claim 1, wherein said wafer has anelectrical resistivity at room temperature of at least 5×10³ Ωcm.
 14. Asilicon carbide single crystal wafer obtained by processing andpolishing a silicon carbide single crystal as set forth in claim 1,wherein said wafer has an electrical resistivity at room temperature ofnot less than 1×10⁵ Ωcm.
 15. A silicon carbide single crystal wafer asset forth in claim 13, wherein said silicon carbide single crystal waferat room temperature is a single polytype of 3C, 4H, or 6H.
 16. A siliconcarbide single crystal wafer as set forth in claim 13, wherein saidsilicon carbide single crystal wafer is comprised of a single polytypeof 4H.
 17. A silicon carbide single crystal as set forth in claim 13,wherein said wafer has a size of at least 50 mm.
 18. A silicon carbidesingle crystal as set forth in claim 13, wherein said wafer has a sizeof at least 100 mm.
 19. An epitaxial wafer comprised of a siliconcarbide single crystal as set forth in claim 13 on the surface of whicha silicon carbide thin film is grown.
 20. An epitaxial wafer comprisedof a silicon carbide single crystal as set forth in claim 13 on thesurface of which a gallium nitride, aluminum nitride, or indium nitridethin film or mixed crystal thin film of the same is grown.
 21. A methodof production of a silicon carbide single crystal by using a sublimationrecrystallization method using a seed crystal to grown a single crystal,said method of production of a silicon carbide single crystalcharacterized by using a sublimating material comprised of siliconcarbide and vanadium or a vanadium compound in a mixture and using forthe crystal growth a graphite crucible having a nitrogen concentrationof not more than 50 ppm as measured by an inert gas fusion thermalconductivity method.
 22. A method of production of a silicon carbidesingle crystal as set forth in claim 21, wherein said graphite cruciblehas a nitrogen concentration of not more than 20 ppm.
 23. A method ofproduction of a silicon carbide single crystal as set forth in claim 21,wherein said graphite crucible has a nitrogen concentration of not morethan 10 ppm.
 24. A method of production of a silicon carbide singlecrystal as set forth in claim 21, wherein said graphite crucible is agraphite crucible treated for purification by being held in an inert gasatmosphere of a pressure of not more than 1.3 Pa at a temperature of1400° C. or more for 10 hours to less than 120 hours.
 25. A method ofproduction of a silicon carbide single crystal as set forth in claim 21,further comprising charging the graphite crucible with a material powdermainly comprised of silicon carbide and, in that state, treating thegraphite crucible for purification by holding it in an inert gasatmosphere at a pressure of not more than 1.3 Pa at a temperature of1400 to 1800° C. for 10 hours to less than 120 hours, placing saidgraphite crucible and seed crystal in an inert gas atmosphere adjustedin pressure to 1.3×10² to 1.3×10⁴ Pa, and heating to 2000° C. or more,then starting crystal growth.
 26. A method of production of a siliconcarbide single crystal as set forth in claim 24, wherein saidpurification treatment is performed at a pressure of 1.3×10⁻¹ Pa orless.
 27. A method of production of a silicon carbide single crystal asset forth in claim 24, wherein said purification treatment is performedat a pressure of 6.5×10⁻² Pa or less.
 28. A method of production of asilicon carbide single crystal as set forth in claim 24, wherein aftersaid purification treatment, said graphite crucible is used for crystalgrowth without being exposed to the atmosphere.